PIN diode attenuator circuits are used extensively in AGC and RF leveling circuits. Typical applications are found in the receiver front-end of broadband communication devices such as Microwave Radio Link equipment, cable or optical-fiber TV, wireless CDMA, etc.
The PIN diode attenuators are well known in the art and may take many forms, ranging from a simple series or shunt mounted diode acting as a lossy reflective switch, to a more complex structure that maintains a constant matched input impedance across the full dynamic range of the attenuator.
In front-end AGC circuits, PIN diode attenuators are extensively employed in conjunction with a control feedback architecture to assure a constant power level at the output of the circuitry.
Although there are other known methods for providing AGC functions, the PIN diode approach generally results in lower power drain, less frequency pulling, lower RF signal distortion and is also less expensive.
A popular attenuator design utilized over a wide frequency range is the Pi (or π) network topology. Benefits of such design are broadband constant impedance and wide dynamic input power range, besides being a relatively inexpensive solution. The PIN diode is used as a current-controlled resistance component in the Pi network providing a variable and controlled RF attenuator.
A typical AGC configuration, based upon PIN-diode attenuator, is shown in FIGS. 1 and 2.
The attenuator 1 comprises three PIN diodes D1, D2, D3 in a Pi topology, and the attenuation level can be changed by a modulation of the forward current of the diodes. The control feedback is easily performed by using an integrator stage 2 that drives the attenuation level as a function of an output power error signal 3. The output power error signal 3 can be obtained by using a directional coupler 4 at the RF output of the attenuator 1 and a power detector 5 connected to the directional coupler 4, in order to generate a power detection signal 6 which is compared to a reference 7 through a comparator 8.
Modulation of the attenuator control signal is commonly performed by driving a BJT transistor stage acting as a simple voltage to current converter.
The main disadvantage of the existing solutions is the strong dependence, for a typical AGC circuit based upon PIN diode attenuator, of the control loop bandwidth versus the input power level. This is due to the intrinsic characteristic of the diode resistance, which varies inversely to its forward current.
In applications where a wide input power range is requested, the control bandwidth spread values could span over more than a decade.
In order to provide an example of such bandwidth spread, reference is made to FIG. 2, which shows a known front-end PIN diode AGC circuit 10 particularly for a high capacity QPSK-QAM microwave radio link modem. Such circuit 10 is a practical implementation of the block diagram of FIG. 1.
The AGC circuit 10 comprises an RF input 11, for instance in the form of a coaxial connector, which is adapted to receive signals over a wide frequency band with a wide power level spread, e.g. within 25-30 dB. The input impedance is approximately 50 ohms.
The selected signal at the input 11 is coupled to a Pi attenuator 1. In particular, the RF input 11 is coupled via a capacitor C1 to the anode of the PIN diode D2, whose anode is connected to the cathode of the PIN diode D1. The cathode of D2, instead, is connected to the cathode of the PIN diode D3, so that diodes D1, D2 and D3 form a known Pi attenuator.
The anode and cathode of PIN diode D2 are respectively connected to ground by means of resistors R1 and R4. The anodes of D1 and D3 are polarized by resistors R2 and R3, respectively, to a bias point BP. Bias point BP is also shunted to ground by a resistor R5 and is connected, through a resistor R6, to the collector of a modulation transistor Q1 acting as modulator of the attenuator control signal and as a voltage to current converter.
The transistor Q1 may be a BJT transistor. The collector of Q1 is biased via R7 to a power supply Vcc, while the emitter of transistor Q1 is coupled to the junction between capacitor C1 and the anode of D2 by means of a choke L1.
The base of Q1, instead, is connected to the output of the integrator stage 2. As a consequence, the voltage on the base of transistor Q1 allows to modulate the RF attenuation by means of the PIN diode attenuator 1.
The AGC circuit 10 further comprises an RF output 12, which is connected to the output of the Pi attenuator 1. The directional coupler 4 is provided at the RF output 12 for dropping the RF output signal and forwarding it as a feedback signal to the RF power detector 5, which is a known device configured to supply an output voltage proportional to its power input.
The signal from the power detector 5, in known AGC circuits, is input through a resistor R9 to the integrator stage 2, which comprises inverting circuitry. In particular, the integrator stage 2 includes an operational amplifier 13 having a reference voltage source Vref at its non-inverting input and a capacitor C5 connecting the output to the inverting input. An integrator resistor R9 connects the power detector output to the inverting input of the operational amplifier 13.
The typical frequency bandwidth of a receiver front-end PIN-diode AGC circuit such as the circuit of FIG. 2 is about few hundreds kilohertz when employed in a microwave radio link.
Due to the physical law governing the resistance channel of a PIN diode versus its forward current Id, the dependence of the AGC bandwidth against the AGC attenuation level is strongly variable. In fact, with reference to FIGS. 3 and 4, the main pole that defines the frequency bandwidth of the circuit 10 is given, besides by the serial resistor R9 and the feedback capacitor C5 of the integrator stage 2, by the transfer function of the PIN diode attenuator 1.
In fact, as it is known, the channel resistance Rd of a PIN diode depends on its forward current according to the following relationship:
            R      d        ∝                  R        min            +              A                  I          d          B                      ,where Rmin is the minimum forward resistance whilst A and B are constant parameters to shape the real function of Rd(Id). Considering that the B value for conventional PIN diodes is around 1, the diode resistance can be considered substantially inversely proportional to its forward diode current, as shown in FIG. 3, namely
      R    d    ∝            1              I        d              .  
Referring to FIG. 2, it is noted that the attenuation transfer function (hereinafter called ATT[dB]) of the Pi network (D1, D2 and D3) and the base voltage on Q1 (called here Vd) feature the same relationship between Rd and Id, namely
      ATT    ⁡          [      dB      ]        ∝            1              V        d              .  
Considering that the time constant of the integrator 2 is τ=R9·C5 and is set to be lower than the other time constants present in the circuit 10, it can be shown that the main pole of the conventional circuit of FIG. 1 is given by the integrator parameters and by the attenuation control voltage Vd which is fed back to the attenuator. More precisely, the circuit bandwidth at −3 dB, hereinafter called BW−3dB, can be found to be
                              BW                                    -              3                        ⁢                                                  ⁢            dB                          ∝                  1                                    R              9                        ·                          C              5                        ·                          V              d                                                          [        1        ]            
Due to the BW−3dB dependency on Vd, the known AGC circuit bandwidth versus the attenuation level exhibits a typical trend as the one shown in FIG. 4, which reports a measured behavior of the known AGC front-end circuit 10 of FIG. 2.
As it is noted, the bandwidth spread results more than 100-120% around the mean value, over an attenuation range of 25 dB.
This behavior may not be acceptable in systems, such as Microwave Radio Link applications, where the receiver front-end AGC response should be as linear as possible when changing the input power level.
In radio links systems the above problem causes low performance in terms of flat fading tolerance, especially at low attenuation level of the PIN diodes network.
Some drawbacks are also present when dynamic modulation features are implemented in the equipment. In other words, fast enough variations of the receiver input power cause faults and errors of the radio link.
In order to solve this drawback it is often needed to use a plurality of AGC stages arranged in a cascade, for achieving an acceptably wide input power range while keeping the frequency response as flat as possible. Unfortunately, these types of workarounds, based on multiple cascaded AGC stages, are quite expensive, are often subject to stability problems in the regions where the control ranges overlap and are difficult to integrate in a single chip.
Another possibility for partially overcoming the above drawback may be to apply a strong limitation on the AGC circuit in term of input power range, but this solution is not acceptable especially in wireless applications, because this would affect the receiver input flexibility.